Ubuntu: If you're running Ubuntu on a laptop, and you want to disable the touchpad when you have it connected to an external keyboard or mouse, Touchpad-Indicator is a menubar applet that can help.

The applet is fairly simple, but it works as advertised. You can hide the indicator once you've toggled a setting, or use it to enable or disable the touchpad at will. Alternatively, you can tell the applet to disable the touchpad only when an external mouse is connected. Set the applet to run when your system starts and it'll always be in the menubar, ready for you to use when you need it.